The invention relates to the modification of water flows, particularly at bifurcations in water channels. Such bifurcations occur in natural river channels and tidal waterways as well as in artificial basins, docks, canals etc.
It has been observed in nature that in many circumstances an eddy current is generated as a result of flow separation immediately within the entrance to a bifurcation (see FIG. 1) and leads to the deposition of sediment. In uni-directional flows, i.e. rivers, the water level changes in height only slowly and such eddy systems, in which the flow rotates about a vertical axis, are set up at bifurcations due to flow separations induced by sharp changes in the orientation of the bank. Such separations are enhanced by velocity gradients between the water in the river channel passing the entrance to the branch and the branch itself, where the velocity, in the case of blind-ending branches, may be zero.
In simple river channels with no tidal effect there is no superimposed advection of water through the eddy system to fill the branch. In tidal waters eddy rotation is supplemented by advection of water into the basin on a rising tide. In either case, a portion of the water drawn into the eddy may eventually escape and enter the side branch whilst another portion may escape and return to the main stream.
In bidirectional or tidal flows, the water level changes in height regularly and in relative terms, rapidly. On rising stages of the tide in these situations it has been observed that a quantity of water larger than that strictly necessary to fill a branch basin at high tide may be exchanged at the entrance to the bifurcation. This implies that the eddy is both drawing in water to fill the basin on the rising tide and also exchanging water back into the main flow. The quantity of water exchanged is difficult to estimate, but in some cases may be two or three times the tidal volume of the basin. (The tidal volume is the surface area multiplied by the tidal range). The strength and persistence of the eddy and also the volume of water which passes through the eddy contribute to its efficiency in trapping and depositing sediment.
In blind-ending bifurcations, such as harbour basins in tidal waters, the importance of the eddy current depends in part on the size and extent of the harbour basin. In small harbour basins the eddy current may occupy the whole area of the basin, whilst in larger basins the eddy may occupy only the entrance to the basin.
The rotary motion in such eddy currents may, in itself, be undesirable in that the strong cross-currents induced may present control difficulties for vessels, of necessity moving slowly whilst entering the branch, but it is also known that large quantities of sediment frequently accumulate on the channel bed beneath such eddy currents. Deposition arises as a result of the so-called "tea-cup effect": as a result of the pressure gradients and the velocity distribution within a rotating eddy, sediment is drawn into its axial region. Such sediment cannot readily escape from the centre of the eddy with the result that a certain proportion of it is deposited. In small harbour basins, with or without a tidal influence, where the eddy fills the entire basin, the entirety of the sedimentation occurring arises from this so-called tea-cup effect. In large harbour basins, with a tidal influence, it has been observed that up to 50% of the sedimentation occurring arises from the tea-cup effect and deposition is concentrated just within the harbour entrance. At waterway bifurcations without blind-ends, such as the branches of rivers or canals, the tea-cup effect is still present and leads to such sediment as is deposited being concentrated in the zone beneath the eddy.
Sedimentation caused by eddy currents in any of the cases described above and in the quantities specified results in high costs for dredging and maintaining navigable waterways. Costs are enhanced when such material is highly contaminated by pollutants and has to be transported long distances or placed in special containment or spoil-disposal areas ashore.